This application requests support for research on two fundamental and widespread behavioral phenomena, choice and learning. Choice is a consequence of behavioral hierarchies (Sherrington, 1960; Tinbergen, 1950, 1951)-- the organization of behavioral acts into dominance relationships which ensure that under specific circumstances the "correct" behavior pattern takes priority over all others. Learning is one of the most important and universal forms of behavioral plasticity. Both choice and learning have for the first time been demonstrated in an animal which has a relatively simple, analyzable nervous system, the gastropod mollusk Pleurobranchaea californica (David and Mpitsos, 1971, see Appendix I; Mpitsos and Davis, studies on learning described in the application and in preparation). An interrelated series of experiments aimed at determining the neuronal substrates of choice and learning in Pleurobranchaea is detailed in this application. Experiments on choice will test the hypothesis that behavioral hierarchies result from inhibitory interactions between identified networks of neurons controlling different individual acts of behavior--notably feeding and righting. Experiments on learning, which also involve the feeding behavior, are described in two parts. First, data from an extensive series of experiments are presented showing that Pleurobranchaea is capable of rapid associative learning (classical reward and avoidance conditioning of feeding behavior). These behavioral experiments will be continued and expanded as part of the proposed research. Second, a correlated program of neurophysiological and histological research has been initiated and will be continued with the aim of identifying the cause of learning on the level of single, identified nerve cells. The proposed research program offers an excellent, unprecedented and unique opportunity to achieve a rigorous understanding of general cellular mechanisms underlying choice and learning, using a simple analyzable nervous system.